1. Field Of The Invention
The present invention relates to power converters (i) using solid state devices to switch currents at high frequency and (ii) functioning as virtual transformers of electrical potentials. The present invention particularly concerns transistorized phase-controlled reversible power converters that present a controllable counter electromotive force (emf) to a source of electrical power that is connected to such power converters.
1.1 General State Of The Art In power Conversion
Modern switching power supplies may be classified in three types. For the past twenty years the dominant power supply design has been based on pulse-width modulation (PWM). In pulse width modulation the amount of time that an input voltage potential is connected to a power transformer is varied in order to keep the output voltage of a power supply constant over varying operating conditions.
Recently, resonant-mode power supplies have come into commercial production. In a resonant-mode power supply energy stored in an inductor is transferred to a capacitor and back again at a rate that is determined by the relative impedance of the components. In a resonant-mode power supply, energy does not pass directly in pulse waveforms to the transformer, as in the pulse-width modulated type. Instead, the incoming current and voltage are stored alternately in an inductor, and in a capacitor, that shape them into sinusoidal waveforms that drive the power transformer. The quasi-resonant supply is the most common type of resonant-mode power supply. In this type of resonant-mode power supply a power switch controls the repetition of the resonant cycles, making the transfer of energy discontinuous.
Therefore both the pulse-width modulated and the resonant-mode power supply regulate the time that a power switch is on and off in order to keep the output voltage constant. The difference between the two types of supplies is that a resonant-mode power supply regulates the output voltage by varying the frequency of the on-off switching, whereas a pulse-width-modulated power supply varies the duty cycle of the on-off switching. Generally feedback control loops are used to detect when adjustments in the output voltage are necessary, and a switching controller is used to carry out the required changes to switching control.
A third, relatively uncommercialized, type of power conversion is called phase-controlled high-frequency power conversion. In phase-controlled power converters the phase of the switching waveforms--which waveforms are simultaneously applied to pairs of switches on each of the primary and secondary sides of a power isolation transformer--are varied in order to control the conversion between the input and output voltage potentials. Power conversion/power converters in accordance with this principle are sometimes called "four quadrant power conversion/converters". The present and related inventions will be seen to preferably be embodied in phase-controlled power converters, and to substantially overcome certain problems that have previously plagued this technology.
Pioneering work on phase-controlled power conversion is attributed to William McMurray of General Electric Company. McMurray's invention is, for example, the subject of seminal United States patent Ser. No. 3,517,300 for POWER CONVERTER CIRCUITS HAVING A HIGH FREQUENCY LINK. McMurray's 1970 patent shows several single phase solid state power converter circuits each having a single high frequency transformer link. Each of the transformer's two windings are connected respectively to (i) a load, and (ii) a d.c., or low frequency a.c., power source, through an associated inverter configuration switching circuit. Each switching circuit employs inverse-parallel pairs of controlled turn-off switches, such as transistors or gate turn-off silicon controlled rectifiers (SCR's). Filter means are connected across the input and output terminals of the power converter circuits. (In the present invention, a filter including an inductor is commonly used with a switching circuit that is variable in its coupling. The combination of the switching circuit and its filter including the inductor is called a "variable port" A switching circuit alone, without a filter or inductor, is called a "fixed port". It is used to directly connect a transformer winding to the terminals of the port under fixed phase switching control, or drive.)
By synchronously rendering conductive one switching device in each of the primary and secondary circuits of the transformer, and by alternately rendering conductive another device in each switching circuit, McMurray's power converters function to couple an input voltage potential to a high frequency waveform, transform the high frequency waveform, and reconstruct from the high frequency waveform a voltage potential at the output terminals. This reconstruction of a relatively low frequency s.a.c. or a d.c. waveform from a relatively higher frequency waveform within the transformer is called "rectification", or "demodulation". A wide range of output voltage control is possible by phase shifting the turn-on of the switching devices on one side of the converter's transformer with respect to the turn-on of the switching devices on the other side. The converter may effect current limiting, current interruption, current regulation, and voltage regulation.
The McMurray power conversion/power converter invention has been the subject of continuing interest because of, among other reasons, (i) its extreme flexibility in converting electrical power potentials having diverse voltage waveforms, and (ii) its efficient use of solid state switching components (transistors or SCR's).
Unfortunately, McMurray's original circuit suffers from efficiency and reliability problems due to high current or high voltage surges that may occur during commutation. This problem is discussed in detail in section 1.5, following. As of 1989, this problem is still being addressed. For example, reference the article PHASE-CONTROLLED D.C.-A.C. CONVERTER WITH HIGH FREQUENCY SWITCHING by Harada, et al. appearing in IEEE Transactions on Power Electronics for October, 1988 at pages 406-411.
In the Harada, et al. article the switching-induced voltage surge during commutation is stated to be reduced by virtue of dividing the filter reactance into two parts. This division, which requires extra components, is directed to preventing an unwanted occurrence of currents at bipolar switch pairs performing waveform rectification resultantly from the recovery times of isolation diodes.
The present invention will be seen not to employ isolation diodes to perform rectification. The present invention will be seen to instead perform rectification of (i) a high frequency waveform that is within the windings of the power converter's transformer into (ii) a d.c., or low frequency a.c., waveform of an electric potential applied to, or produced by, the power converter, solely by switching in transistors, and totally without diodes. Accordingly, there are no unwanted currents at the switches due to rectification. (There may be overcurrents at the switches for other reasons, as explained in section 1.4, following. However, the switches of power converters in accordance with the present invention will be seen to connect directly through filters to external sources of voltage potential--an extremely rare form of power converter connection--and not to connect through diodes in order to perform rectification.)
Furthermore, the Harada, et al. converter employs a center-tap connection to the isolation power transformer. This form of transformer connection means that, at most, the transformer utilization efficiency will be limited to 50%--i.e., at any one time one-half of the center-tapped transformer winding carries no current. Even when connection to a transformer winding is across the entire winding by a switching circuit in a bridge configuration, the transformer will not be utilized to its maximum efficiency unless it constantly carrier the maximum voltage excitation waveform of which the transformer is capable. A sinusoidal excitation waveform within a transformer gives, by definition, less than 100% utilization efficiency of the transformer because the waveform is not always at its maximum voltage.
The present invention will be seen to strongly prefer connecting switching circuits in a full bridge configuration across an entire transformer winding, and to constantly excite the transformer with maximum voltage. Power converters in accordance with the present invention will therefore be seen to enjoy a transformer utilization efficiency of 100%.
Even though the individual switching control signals applied to the individual switches (transistors or SCR's) of the switching circuits of McMurray and Harada, et al. may be constant amplitude square waves--and thus admirably suitable to be transformer coupled to the switches--these switching control signals produce, in aggregate, a high frequency a.c. waveform within the power transformer that is not optimal. This a.c. waveform is not optimal because it is undesirably both (i) carries an amplitude modulation that reflects the low frequency of the port voltage, and therefore, by its modulation influences all conversions at other ports and is (ii) customized to a particular type, and to the associated particular voltage waveforms, of the particular power conversion that is being performed. An amplitude-modulated high frequency a.c. waveform within the power transformer carrying the low frequency of a port's voltage is not independent of that port, and thus precludes multiple independent port control. The high frequency amplitude-modulated a.c. waveform within the power transformer is (ii) customized to a particular conversion. It will, by definition, be inapplicable to other conversions, and will prevent such additional conversions from being performed simultaneously, and in parallel, by use of a single power transformer. Power converters in accordance with the present invention will be seen to produce a transformer excitation that is independent of any port's time-varying voltage. Accordingly, these power converters will be seen to support multiple ports, and multiple port control, that is impossible in prior art phase-controlled power conversion/converters.
The present and related inventions will be seen to contemplate power conversion from sources of input power--particularly including alternating current power--with (i) no low frequency component in the transformer excitation waveform, and (ii) a substantially constant amplitude transformer excitation waveform that is universally suitable for diverse power conversions. The excitation waveform will also be of an equal duty cycle, which makes along with the substantially constant amplitude that the excitation waveform is a square wave. It will be seen that use of such an excitation waveform within the transformer is the antithesis of customization, and that diverse useful power conversions can be performed simultaneously and concurrently using but a single power transformer if the transformer excitation waveform is an equal duty cycle substantially constant amplitude square wave.
The present and related inventions will also be seen to (i) substantially overcome previous commutation problems, (ii) much enlarge the scope of application, and (iii) improve the efficiency of phase-controlled power conversion. The aggregate result of these improvements will be sufficient so as to warrant comparison of phase-controlled power conversion in accordance with the present and related inventions with the best power conversion efficiency and densities previously obtainable. Accordingly, the following two sections respectively further discuss (i) density, and (ii) efficiency, aspects of previous power conversion in order that the quantitative performance of the present invention, as later explained, may be more clearly appreciated.
1.2 Size And Density Of Previous Power Conversion Circuits
The present and related inventions will be seen to more significantly improve to the efficiency of power conversion--the state of the art relative to which is discussed in the following section--than minimize the size, and increase the density, of power conversion circuits. Nonetheless, the current state of the art in power supply size, and density, is discussed in this section because the size and density of power converters in accordance with the present invention will be seen to rival the best existing performance in this area. Moreover, power converters in accordance with the present invention have great potential to improve, with advances in semiconductor (as opposed to magnetic) component technology, beyond their current sizes and densities.
There is a present need to simplify the design and construction of low voltage electronic power supplies. Presently-available power supplies often exceed the remaining volume of the systems powered; a rule of thumb is for the power supply's volume not to exceed 25% of the volume of the system. In order to come down in size and so increase power density, it was previously perceived that any type of power supply must operate at frequencies approaching 1 MHz so that very small transformers and output filters may be used. The higher the switching frequency, the smaller the transformer core need be--but the larger the switching losses. In pulse-width modulated power supplies both high voltages and currents coexist during turn-on and turn-off of the power switches, resulting in high switching losses.
The higher the switching frequency, the more a power supply's ripple and transient response are improved. If switching frequency is increased while the components of an output signal filter are kept constant, then the power supply's ripple will be much smaller. Conversely, if the components of the output signal filter are made smaller, then the bandwidth of the feedback control loop, which senses the power supply's output and which signals the controller to make any necessary changes, can be widened dramatically in order to improve the transient response while still providing low ripple and adequate stability.
Resonant-mode power supplies are the current leaders in size, packaging, density and efficiency (this latter factor being more extensively discussed in the following section). Typical prototype resonant-mode power supplies have power densities of 35 watts per cubic inch (573 watts per cubic centimeter). The most advanced resonant-mode power supplies have densities exceeding 50 W/in.sup.3. In contrast, the typical density of an advanced pulse-width modulated supply is about 10W/in..sup.3. Even if pulse-width modulated supplies could go higher in frequency, switching losses would bring their efficiency down to about 60 percent at 1 MHz. Because of their minimal switching losses, resonant-mode supplies operate with more than 80 percent efficiency above 1 MHz.
Up to about 2 kw maximum output, phase-controlled power converters (a general term including input-to-output power supplies in accordance with the present invention) will be seen to have competitive power densities, and efficiencies of up to 95%. Appreciation of exactly how power converters in accordance with the present invention achieve these levels, however, requires a more detailed understanding of previous power supplies than the simplistic association of switching frequency with power supply size and density. In particular, and recalling that the size of the magnetic components within an electronic power supply is related to the oscillator frequency, it will later be disclosed that power converters in accordance with the present invention commonly operate at a switching frequency of approximately 80 kHz (although this rate will probably be extended to the region of 120 kHz within five years). At first impression, this rate seems slow relative even to pulse-width modulated power supplies, which regularly operate at frequencies up to 100 kHz, as well as to resonant-mode power supplies which have operated, in experimental form, at frequencies as high as 10 MHz.
In order to understand how power conversion in accordance with the present invention will be seen to use magnetic components of about the same size, and in approximately the same density, as existing PWM and resonant-mode power supplies, a detail knowledge of several factors other than frequency of operation affecting the size of magnetic components in power supplies is required. First, power conversion in accordance with the present invention will be seen to extend to kilowatt (kw) and higher power levels. PWM power supplies operating at this power level typically operate with a switching frequency in the order of 20 kHz-30 kHz. Therefore, for its power level, power conversion in accordance with the present and related inventions at kw power levels is at a somewhat improved frequency over what is typical in the previous art.
Second, power conversion/power converters in accordance with the present invention will be seen to couple (convert from and to) voltage potentials having an a.c. component through the use of three-state demodulated waveforms. These waveforms may be rectified to produce high-quality a.c. waveforms in filter components that are of reduced size compared to filter components used to rectify the two-state waveforms that are commonly produced by PWM and resonant-mode power supplies. Three-state-demodulated signals generally contain only one-half the harmonic power density of two-state demodulated signals commonly produced in the prior art. Accordingly, the size of the filter components, particularly including inductors, that are required to filter these signals is significantly reduced by ratios up to one half.
Third, power conversions/power converters in accordance with the present invention will be seen to make maximally efficient 100% utilization of a high frequency power transformer. Because this optimal 100% utilization figure cannot be improved upon, a power converter in accordance with the present and related inventions will be seen to derive as much efficiency from its primary magnetic component--the high-frequency power transformer link--as is absolutely possible. Previous power supplies of both the PWM, resonant mode, and phase-modulated types make only such utilization efficiency of the power transformer as the excitation waveform(s) therein approach (i) an equal (50%) duty cycle and (ii) a constant, maximum, amplitude. In other words, only square wave excitation is optimal. Non-pulse, sinusoidal, transformer waveforms do not represent an optimal utilization of the transformer. A pulsed, or pulse-width-modulated, waveform is efficient only to the extent that it is balanced. For example, A PWM waveform that spends 75% in one state and 25% in the other state is balanced only to the extent of 25%, and uses the transformer with 25% utilization efficiency. Resonant mode power supplies that are purposely inactive for energy transfer through the transformer during portions of a cycle make inefficient utilization of the transformer. Finally, even a phase modulated power supply will be less than 50% efficient in utilization of an entire transformer if its switches are connected to its transformer in a center-tap configuration where, at any one time, one-half of each transformer winding carries no current.
Fourth, the size of the magnetic components can be traded off for efficiency of operation in power conversions/power converters in accordance with the present and related inventions. Generally it is neither advisable, nor required, to trade off efficiency in order to minimize the size/maximize the density of power conversion/power converters. However, power conversions/power converters in accordance with the present and related inventions will be seen to exhibit marked improvement in efficiency over power supplies of the prior art. Accordingly, if the 95% efficiency of power conversions/power converters in accordance with the present and related inventions is reduced to the maximum efficiency levels--approximately 70-80%--of the prior art, then the size and power density of the power converters is generally superior to such prior art--especially at kilowatt power output levels.
1.3 Efficiency of Power Conversion
Efficiency in previous power conversion is related to the design of the power switch. A zero-voltage switch turns on and off only when the voltage waveform is at zero, and a zero-current switch turns on and off only when the current waveform is at zero. Either type of switch greatly reduces the switching losses in a power supply.
In pulse-width modulated (PWM) power supplies high voltages and currents both exist during turn-on and turn-off of the power switches, resulting in high switching losses.
Resonant-mode power supplies attempt to overcome this problem. The first resonant-mode power supply design to be developed, which is used in most of the resonant-mode power supplies now in production, is a zero-current switching topology. In this configuration energy is transferred only when the power switch is on, so that frequency of switching increases with the load. In order to increase the energy transfer, the power switch must be turned on more frequently. If a full-cycle switch is used, then tank energy storage and recycling may, however, serve to greatly reduce the frequency change. Parasitic junction capacity in the switches causes some switching losses when the power switch turns on, and also requires a high gate current. This generally restricts the operation of zero-current switching topology resonant-mode power supplies to about 1 MHz.
Experimental zero-voltage switching topologies for resonant-mode power supplies are also known. In this topology the frequency of switching decreases as the load increases to a minimum that is specified in the power supply design. This occurs because an increase in the load causes the power switch, instead of being turned on more often as in the zero-current switch topology, to be turned on less often for longer periods of time. The maximum switching frequency, which may be high as 20 MHz, occurs at the minimum load. A major difficulty when zero-voltage switching topology resonant-mode power supplies is the very large voltage stresses that occur in the power supply components.
The two approaches to resonant-mode power supply design--zero-current or zero-voltage switching topologies--give the best currently known efficiencies of power conversion. These efficiencies are on the order of 70%-80% depending upon power levels, voltages, currents, and components.
The present invention will be seen to reject the strategy of switching at zero-current or at zero voltage. In this regard, power converters in accordance with the present invention will be seen to switch non-zero current, non-zero voltage, electrical potentials--similarly to switching occurring at a PWM power supply. At first impression this seems a dubious approach to realizing high efficiency through low switching losses. However, the present invention will be seen to minimize losses occurring during switching by the strategy of making switching occur extremely fast, on the order of nanoseconds. Because the amount of energy lost during switching is a function of the rapidity of such switching, ultra high speed switching produces switching losses that are lower than those commonly encountered in either the prior art PWM and resonant-mode power supplies even though the electric potentials switched are non-zero current, non-zero voltage.
The efficiency of power conversion/power converters in accordance with the present invention will be seen to be on the order of 95%, which is considerably higher than the best obtainable--even by certain experimental quasi-resonant resonant-mode power supplies--in the prior art.
1.4 Limitations Of Prior Art At A Single Phase Line Interface
Among the most serious limitations of prior PWM and resonant power supplies is that such power supplies have no good way to interface with a single phase AC line as a source of power. PWM and resonant-mode power supplies require a DC source. Accordingly, the line voltage is almost universally (i) diode bridge-rectified and (ii) filtered by a capacitor filter. The power factor (the ratio of apparent power to real power) presented to the line is low, typically 0.6, because all of the current is drawn at the tips of the voltage waveform. Use of power supplies having less than unity power factor is so prevalent in military applications such as shipboard applications, or in buildings housing extensive numbers of small computers, that the line voltage itself is severely distorted near the waveform peaks. Power conversion in accordance with the present and related inventions will be seen to totally and directly solve this problem by presenting a perfect, unitary, power factor to a single phase a.c. line interface.
Most previous power supply circuits draw current from a single phase a.c. line in large surges at the voltage peak of each half cycle. This current charges a holding capacitor(s) to a voltage nearly equal to the peak voltage of the input line voltage. The stored voltage dissipates a small amount into the load between peaks. These large current surges are undesirable for several reasons.
First, the resulting product of RMS amperes and RMS volts (VA) is considerably larger than the power actually consumed by the power supply circuit and its associated loads. This difference is defined as the power factor, which equals the total power consumption divided by the VA's. In accordance with the present and related inventions, the power factor of a power converter is normally controlled to be nearly unity. Thus the needed RMS current required to deliver equal power is reduced considerably. The exact reduction is inversely proportional to the power factor, e.g. 10 amps with a power factor of 0.6 produces the same power as 6 amps with a power factor of unity.
Second, the large current surges typical of previous power supply technology also tend to cause distortion of the input line voltage. More specifically, the current surges tend to flatten the peaks of the input power voltage waveform. This flattening not only tends to reduce the peak voltage to the particular power supply device causing the peak reducing current surges, but to all other power supplies on the same circuit. Since most previous power supplies depend on the peak voltage for normal operation, the effect of a reduced peak voltage is the same as reducing the line voltage.
Third, the large surge currents are destructive to electrolytic capacitors. The life expectancy of those capacitors is a function of the internal power dissipation and time. Higher peak currents cause a proportionally high RMS current in the holding capacitors.
Fourth, the high RMS current typical of previous power supplies makes meaningful fusing difficult in that the designed fault current must be a multiple of the normal operating current, e.g. 1.25 times operating current.
With a unity power factor power converters (power supplies) designs delivering higher power can be operated from the same outlet or circuit. Power converters in accordance with the present and related inventions not only permit control of the power factor under normal operating conditions, but will be seen to also usefully control inrush current when initially energized.
1.5 Broadband and Narrowband Conducted and Radiated Noise
Existing PWM, resonant mode, phase-modulated and other "switching" power supplies are notoriously noisy. The high frequency switching, based on a clock oscillator, creates both broadband and narrowband conducted and radiated noise. Because the conversion of a.c. power by switching has previously required that the duration, frequency, and/or phase angle of the switching control waveforms should be related to the waveform of the input a.c. power that is converted, the necessary relationship has been referenced relative to an oscillator clock for which the frequency is fixed. This fixed clock frequency, and the switching control signals and actual switching derived therefrom and timed relative thereto, produce sharp peaks in a plot of conducted and/or radiated electromagnetic noise versus frequency. These peaks are the "signature" of the power supply. They are especially undesirable in military applications where they contribute to the electromagnetic signature, and to the potential location and/or identification, of a platform such as a plane or ship upon which the switching power supply is located.
Power converters in accordance with the present invention will be seen to perform switching at high frequencies that radiate well. However, it will also be seen that the switching is extremely fast, on the order of nanoseconds, and produces only modest noise energy transients. Therefore the radiative energy is modest. Moreover, it will be seen that the frequency of a clock oscillator in the power converter of the present invention--from which power converter all switching waveforms of both fixed and variable phase are derived--is completely independent of the waveforms switched, and may actually be swept or even randomized in frequency. Randomizing the frequency of the fundamental clock oscillator from which all switching signals are derived randomizes the frequencies of the collective switching signals, and thereby essentially reduces both broadband and narrowband conducted and radiated noise to white noise in the band over which frequency is varied. When this band is many kHz in width, as will be seen to be the case for power converters in accordance with the present invention, then the noise emissions from such power converters will be very low and extremely difficult, or even impossible, to detect.
1.6 Previous Power Converter Circuits Having A High Frequency Link And Functioning As Virtual Transformers of Electrical Potentials
Power converters, and power conversion, in accordance with the present and related inventions is related to the phase-controlled power converters/ conversion pioneered by McMurray. As previously mentioned in section 1.1, the power converters of McMurray, and their progeny, suffer greatly from destructive high current or high voltage surges occurring during commutation. It is useful to understand the detail nature of this problem in order that its solution by the present invention may be best appreciated.
It is known from McMurray to construct power converters by connecting form A switches in pairs and by controlling the switching of each form A switch by a signal that is the inverse of a signal controlling switching in the other switch of the pair--forming thereby a form C switch. Two form C switches are connected in a bridge circuit across one of a transformer's windings. Because of this connection, and because of the control of the form C switches, power converters after the model of McMurray have been subject to adverse high currents, or high voltage surges, during commutation. Adverse high current results if both form A switches are simultaneously conducting, even if but momentarily, presenting a short circuit to the transformer winding. Conversely, a high voltage surge results if both form A switches are simultaneously non-conducting, even if but momentarily. In this case a transient voltage spike is produced across at least one of the switches. If this voltage spike exceeds the rated voltage of the switch, and if it lasts sufficiently long so as to contain significant energy, then it can cause the switch to fail.
Power converter circuits in accordance with the present invention will be seen to use the remarkable property of Field Effect Transistors (FETs)--including Metal Oxide Semiconductor Field Effect Transistors (MOSFETs) and power MOSFETs--that they can be turned off as fast as they can be turned on. Power MOSFET switching times are typically tens of nanoseconds, and are very short in relation to the periods of both (i) those a.c. power waveforms which are typically converted (typically in the range of 2.5 to 16.6 msec), and also (ii) a higher frequency carrier waveform (typically a 80 KHz waveform of period 12.5 microseconds) that is commonly used in converter circuits in accordance with the present and related inventions.
It is further known in previous phase-controlled power converters after the model of McMurray to use FETs and MOSFETs as unipolar and bipolar bilateral form A switches, and to control these switches by switching control signals that are coupled to the gate of each FET or MOSFET through a switching signal transformer (which switching signal transformer is not the main transformer used for power conversion). It is still further known to connect two such form A switches together in series and to control each switch by a switching signal that is the inverse of the signal controlling the other switch. The two normal and inverse switching signals are coupled through a switching signal transformer, and create from the two form A switches a form C switch. The transformer coupling of the switching control signals is useful for isolation between switching control, and power, sections of a phase-controlled power converter. The FETs or MOSFETs are reliable, solid-state, devices. However, the mere (i) transformer coupling of switching control signals to some particular (ii) FET or MOSFET switch does not, in of itself, control the switching in the collective FET or MOSFET switches of the power converter so as to best eliminate or alleviate current shorts or voltage surges due to commutation.